The proposed research has as its long-term goal the development of a quantitative account of the rules governing discrimination learning and choice behavior in pigeons. The development of discrimination and the nature of generalization gradients will be studied with a go/no-go method and with a method that requires the subject to choose one of two response keys in a successive discrimination situation. A large number of the basic phenomena of "discrimination and generalization" will be reexamined with these methods, e.g. the effects of stimulus similarity on the length of the presolution period in learning and on generalization gradients obtained after discrimination training, and the effects upon these same phenomena of partial reinforcement, reinforcement magnitude, and the presentation probabilities of the discriminative stimuli. Conditional discrimination, and the combination rules governing responding to multidimensional stimuli will also be investigated. The proposed theoretical analysis is based on statistical decision theory and a concept of attention.